Free-cutting non-round steel and method for manufacturing the same

Abstract

The present application relates to the technical field of metal smelting, and relates to a manufacturing method of free-cutting non-rounding steel, which is as follows: in the process of billet continuous casting, the water quantity of two water cooling is controlled to be 0.26 L / Kg; in the process of billet slow cooling: the billet is cooled in a pit at a temperature of greater than or equal to 600 DEG C for more than or equal to 24 hours, and then is heated in a heating furnace at a temperature of greater than or equal to 300 DEG C; in the process of billet heating: the second heating temperature is controlled to be 1220 DEG C plus or minus 20 DEG C, and the soaking temperature is controlled to be 1200 DEG C plus or minus 20 DEG C; in the process of high-pressure water descaling: the high-pressure water descaling pressure is controlled to be 18 plus or minus 1 MPa; in the process of rolling: the cooling water flow is controlled to be 110-130 m 3 / h, so as to reduce stress cracks generated in the process of cold water cooling and cold rolling; the gap between the wear plate of the billet pusher and the rolled piece is controlled to be 20-30 mm at the inlet end and 30-40 mm at the outlet end. The present application reduces the surface stress cracks and scratches of round steel, improves the leakage magnetic initial exploration qualified rate, reduces the round steel rejection rate, shortens the finishing cycle, ensures the finishing delivery period of products, and the like.

Description

Technical Field

[0001] This invention relates to the field of metal smelting technology, and more specifically, to a free-cutting non-rounding steel and a method for manufacturing the same. Background Technology

[0002] The composition of free-machining non-quenched steel, excluding iron, includes: carbon 0.36-0.39%, nitrogen 0.013-0.020%, sulfur 0.04-0.055%, vanadium ≤0.02%, titanium ≤0.01%, silicon 0.54-0.64%, and manganese 1.43-1.5%, etc. Vanadium, titanium, nitrogen, and sulfur are added to medium-carbon manganese steel to dissolve in austenite during heating, as the solid solubility of vanadium, titanium, and nitrogen in austenite decreases with cooling. The trace elements vanadium, titanium, and nitrogen precipitate as fine carbides and nitrides in the initially precipitated ferrite and pearlite. During cooling after hot rolling, vanadium (VN) precipitates rapidly. If the cooling rate is too fast or the phase transformation stress is too high, the precipitates pinning grain boundaries can lead to stress concentration, increasing the risk of surface cracking. Free-machining non-quenched steel is a crack-sensitive steel and is prone to defects such as cracks during continuous casting.

[0003] Free-cutting non-adjustable round steel is used in forgings such as crankshafts and connecting rods. High surface quality is required, with surface defect depths ≤0.2mm. Due to its high nitrogen and sulfur content, and improper process control during billet production, stress cracks and surface scratches easily occur on the round steel surface, resulting in a magnetic flux leakage (MF) test pass rate of less than 70%. For defective materials, finishing grinding wheels are needed to remove the outer layer, which can salvage materials with defects less than 0.5mm in depth. However, the finishing cycle is long, affecting the product's finishing delivery time. Materials with defects greater than 0.5mm cannot meet dimensional tolerance requirements after grinding and must be scrapped, resulting in a scrap rate of 12%, impacting metal yield and increasing production costs. Therefore, it is necessary to propose a new process to solve these problems. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a free-cutting non-adjustable round steel bar and its manufacturing method, thereby reducing surface stress cracks and scratches on the round steel bar, improving the initial pass rate of magnetic flux leakage detection, reducing the scrap rate of the round steel bar, shortening the finishing cycle, and ensuring the finished product delivery time.

[0005] The above-mentioned technical objective of the present invention is achieved through the following technical solution: A method for manufacturing free-cutting non-rounding steel is as follows: During the continuous casting of steel billets, the water volume of the secondary water cooling system is controlled at 0.26 L / Kg; During the slow cooling stage of the billet: the billet is placed in the pit at a slow cooling temperature of ≥600℃ and the slow cooling time is ≥24 hours. Then it is placed in the heating furnace for heating, and the temperature in the heating furnace is ≥300℃. During the billet heating stage: control the secondary heating temperature at 1220℃±20℃, and control the homogenization temperature at 1200±20℃; During the high-pressure water descaling stage: the high-pressure water descaling pressure is controlled at 18±1MPa; During the rolling stage: the cooling water flow rate is 110-130 m³ / h. 3 / h control reduces stress cracks generated during the cold-water-deficient rolling process; The clearance between the wear-resistant plate of the billet pusher and the rolled workpiece should be controlled at 20-30mm at the inlet end and 30-40mm at the outlet end. Increasing the clearance between the wear-resistant plate of the billet pusher and the rolled workpiece can reduce the scratches caused by the wear-resistant plate on the rolled workpiece.

[0006] In one embodiment, during the billet heating stage, the second heating temperature is higher than the homogenization temperature.

[0007] In one embodiment, during the rolling stage, side rollers are added to the billet pusher, with two side rollers installed on each of the front and rear pushers, ensuring that the side rollers are 3-5mm higher than the wear-resistant plate, reducing the contact between the rolled piece and the wear-resistant plate, and thus reducing the scratches caused by the wear-resistant plate on the rolled piece.

[0008] In one embodiment, during the rolling stage, the finished product rolling dimensional tolerance is controlled by controlling the upper limit positive tolerance according to the second group of national standards, increasing the allowance for grinding and peeling, and reducing the scrap rate of round steel.

[0009] In one embodiment, during the rolling stage, the rolling temperature of both the intermediate rolling and finishing rolling is ≥900°C.

[0010] In one embodiment, after rolling is completed, the shearing temperature of the flying shear is controlled to be ≥900°C.

[0011] A free-cutting non-rounding steel is obtained by the above-described manufacturing method.

[0012] In summary, the present invention has the following beneficial effects: This invention employs a series of process improvements, including controlling the water volume ratio in the continuous casting of steel billets, controlling the temperature of the billets during slow cooling before entering the heating furnace, optimizing the heating process, reducing the pressure of high-pressure water descaling, reducing the amount of cooling water in the rolls, reducing the gap between the rolled piece and the wear-resistant plate of the billet mill, and adding side vertical rollers to the tail end of the wear-resistant plate of the billet mill. These improvements reduce stress cracks and scratches on the surface of round steel, improve the initial pass rate of magnetic flux leakage detection, control the finished product according to the national standard group two positive tolerance during rolling, reduce the scrap rate of round steel, shorten the finishing cycle, and ensure the finished product delivery time. This invention can be promoted and applied in similar production lines for special steel. Detailed Implementation

[0013] The present invention will now be described in detail with reference to the embodiments.

[0014] This invention proposes a manufacturing method for free-cutting non-round steel. By adopting a series of process improvement measures, such as controlling the water volume ratio of the continuous casting of steel billets, slow cooling of steel billets before entering the heating furnace, optimizing the heating process, reducing the pressure of high-pressure water descaling, reducing the cooling water of the rolls, reducing the gap between the rolled piece and the wear plate of the billet mill, and adding a side vertical wheel at the tail end of the wear plate of the billet mill, stress cracks and scratches on the surface of the round steel are reduced.

[0015] The manufacturing method of the present invention is as follows: During the continuous casting of steel billets, the specific water volume in the secondary cooling zone is controlled at 0.26 L / kg. In continuous casting, the specific water volume in the secondary cooling zone is a core parameter affecting surface quality. Excessive specific water volume will exacerbate uneven cooling and thermal stress, resulting in surface cracks. At the same time, sulfur combines with manganese (Mn) to form manganese sulfide (MnS) inclusions. When the cooling rate is inappropriate, MnS will agglomerate between dendrites. During subsequent rolling, these inclusions will be elongated, disrupting the continuity of the metal matrix and becoming the source of microcracks, leading to defects such as cracks.

[0016] In existing technologies, the water volume for secondary water cooling is controlled at 0.4 L / kg, which easily leads to cracking defects in the steel billet. During the slow cooling stage of the steel billet: the billet is slowly cooled at a temperature ≥600℃ for ≥24 hours before being placed in the heating furnace at a temperature ≥300℃. Slow cooling for ≥24 hours facilitates stress release in the continuously cast billet. A furnace temperature ≥300℃ prevents the generation of new thermal stress in the preheating section and reduces the heating time of the billet in the brittle zone (600-900℃), thus reducing VN precipitation and preventing surface cracks.

[0017] In the existing technology, since there is no clear specification for the temperature of the billet entering the heating furnace, the temperature of the billet entering the heating furnace is sometimes less than 300°C. The low temperature of the billet entering the furnace will generate new thermal stress during the preheating process in the heating furnace, increase the heating time of the billet in the brittle zone of 600-900°C, which can increase the precipitation of VN and lead to the generation of surface cracks.

[0018] During the billet heating stage: the secondary heating temperature is controlled at 1220℃±20℃, and the homogenization temperature is controlled at 1200±20℃. A secondary heating temperature higher than the homogenization temperature is beneficial for heating uniformity and reduces the generation of thermal stress. A homogenization temperature of 1200℃ is beneficial for the billet to fully dissolve V and N elements, inhibiting grain boundary chain precipitation, reducing the precipitation of VN compounds that could lead to stress cracks, and simultaneously ensuring that the billet remains in a highly ductile state throughout the rolling process, making it less prone to deformation and cracking.

[0019] Preferably, the second heating temperature is higher than the homogenization temperature.

[0020] During the high-pressure water descaling stage: the high-pressure water descaling pressure is controlled at 18±1 MPa to reduce stress cracks that may occur when the cooling water cools the surface of the steel billet during descaling. In existing technologies, the descaling pressure is even higher, typically 23 MPa.

[0021] During the rolling stage: the cooling water flow rate is 110-130 m³ / h. 3 / h control reduces stress cracks generated during the cold-water-deficient rolling process; Furthermore, the gap between the wear-resistant plate of the billet pusher and the rolled piece should be controlled at 20-30mm at the inlet end and 30-40mm at the outlet end. Increasing the gap between the wear-resistant plate of the billet pusher and the rolled piece can reduce the scratches caused by the wear-resistant plate on the rolled piece.

[0022] Furthermore, side rollers are added to the billet rolling mill pusher, with two side rollers installed on each of the front and rear pushers. This ensures that the side rollers are 3-5mm higher than the wear-resistant plate, reducing the contact between the rolled piece and the wear-resistant plate and thus reducing scratches caused by the wear-resistant plate on the rolled piece.

[0023] Furthermore, the dimensional tolerance control of finished rolled products is as follows: the upper limit of the positive tolerance in the second group of national standards is controlled, the allowance for grinding and peeling is increased, and the scrap rate of round steel is reduced.

[0024] Furthermore, during the rolling stage, the rolling temperature of both the intermediate rolling and finishing rolling is ≥900℃.

[0025] Furthermore, after rolling is completed, the shearing temperature of the flying shear is controlled to be ≥900℃.

[0026] It should be noted that this invention achieves the effect of reducing the scrap rate of free-cutting non-round steel only through the optimized coordination of the above-mentioned stages. This invention reduces surface stress cracks and scratches on round steel by adopting a series of process improvement measures, such as controlling the water volume ratio in the continuous casting of billets, controlling the temperature of the billets during slow cooling before entering the heating furnace, optimizing the heating process, reducing the pressure of high-pressure water descaling, reducing the amount of cooling water in the rolls, reducing the gap between the rolled piece and the wear-resistant plate of the billet mill, and adding side vertical rollers to the tail end of the wear-resistant plate of the billet mill. This reduces the surface stress cracks and scratches on the round steel, improves the initial pass rate of magnetic flux leakage detection, and controls the finished product according to the national standard group two positive tolerance during rolling. This reduces the scrap rate of round steel, shortens the finishing cycle, and ensures the finished product delivery time. This invention can be promoted and applied in similar production lines for special steel.

[0027] Based on the above-described method for manufacturing free-cutting non-rounding steel, one specific process of the present invention is as follows: S1. Smelting: A 130-ton converter, LF furnace and RH furnace are used. The RH vacuum treatment time must be maintained at ≥15min, the high vacuum time at ≥10min, the pure degassing time at ≥8min, and the soft argon blowing time at ≥10min to ensure that the inclusions float up fully. S2. Continuous casting: The continuously cast billet adopts a crystallizer and end electromagnetic stirring, which can promote the homogenization of sulfide composition, reduce dendrite segregation, and make the sulfide distribution more uniform. The water volume of the second water cooling is controlled at 0.26 L / Kg. S3. Slow cooling of steel billets: Steel billets at ≥600℃ are put into the pit for slow cooling. After slow cooling time of ≥24 hours, they are put into the heating furnace for heating. The temperature of the heating furnace is ≥300℃. S4. The continuous casting billet 320*425*9000 is heated by a walking beam furnace, with a secondary heating temperature of 1220℃±20℃ and a soaking temperature of 1200±20℃. S5. High-pressure water descaling, with a descaling pressure of 18±1MPa, reduces stress cracks that occur when the cooling water cools the surface of the billet during the descaling process. S6. Rolling is performed using a two-roll reversible rolling mill, with a roll cooling water flow rate of 110-130 m³ / h. 3 / h control reduces stress cracks generated during the cooling and cooling process of the rolled piece. The billet mill rolls 11 passes, with a billet size of 205*205. The gap between the wear-resistant plate and the rolled piece on the billet mill pusher is controlled at 20-30mm at the inlet end and 30-40mm at the outlet end. Increasing the gap between the wear-resistant plate and the rolled piece on the billet mill pusher can reduce the scratches caused by the wear-resistant plate on the rolled piece. Side vertical rollers are added to the billet mill pusher, with two side vertical rollers installed on each of the front and rear pushers. The side vertical rollers are ensured to be 3-5mm higher than the wear-resistant plate to reduce the contact between the rolled piece and the wear-resistant plate, which can reduce the scratches caused by the wear-resistant plate on the rolled piece. S7, 1# hydraulic shearing head, cutting length ≥100mm, to remove end defects of rolled billet; S8, intermediate rolling mill, preferably, the intermediate rolling mill has 6 continuous rolling stands, all of which are arranged alternately horizontally and vertically. The rolling mill consists of φ750x6, all of which are fifth-generation short-stress rolling mills. The rolling temperature is ≥900℃, the round steel specification is Φ110mm ≤ Φ220mm, and the finished product dimensional tolerance is controlled according to the upper limit positive tolerance of the second group of national standards.

[0028] S9. Remove head and tail defects. Maximum shear section: Φ142mm. Shearing temperature: ≥900℃. Φ132mm < round steel specification ≤ Φ142mm. Use No. 1 flying shear for multiple-length segmentation. Round steel specification ≤ Φ110mm. Head cut length ≥ 100mm. Tail cut length ≥ 100mm.

[0029] S10, Precision Rolling, the preferred type, the four consecutive rolling mills of the precision rolling mill are all arranged alternately in horizontal and vertical positions, the mill consists of φ550x4, all of which are fifth-generation short-stress rolling mills, rolling temperature: ≥900℃, Φ70mm ≤ round steel specification ≤Φ105mm, finished product dimensional tolerance is controlled according to the upper limit positive tolerance of the second group of national standards; Specific processes may also include: S11 and No. 2 flying shears have a maximum shearing section of Φ132mm and a shearing temperature of ≥900℃. When the round steel specification is ≤φ132mm, the No. 2 flying shear is used for multiple-length segmentation. S12, Grouping: Group the flying shear multiple rulers; S13. Three Φ1800mm metal saws (saw #1, mobile saw, and saw #2) are used for sawing and segmentation. The sawing temperature is 700℃-850℃. S14, cooling bed, air cooling, straightening; S15. Slowly cool down before entering the pit; pit temperature ≥400℃. S16, slow-cooling material exits the pit; S17, Straightening; S18, Flaw Detection, Magnetic Leakage: 0.2mm; S19. Bundling and storing; S20, Grinding wheel peeling, ¢180 twelve grinding head round steel peeling machine, grinding wheel: ¢500*100*203, for peeling defective materials detected by magnetic flux leakage; S21, Flaw detection: Re-inspection of peeled and ground round steel bars; Magnetic leakage: 0.2mm. S22. Bundled and stored.

[0030] The technical solution of the present invention will be described below with reference to specific embodiments.

[0031] The following examples and comparative examples use the production of 100mm diameter round bars from free-machining non-rolled steel billets as an example. Specifically, the steps include the following: Smelting: A 130-ton converter, LF furnace and RH furnace are used. The RH vacuum treatment time must be maintained at ≥15min, the high vacuum time at ≥10min, the pure degassing time at ≥8min, and the soft argon blowing time at ≥10min to ensure that the inclusions float up fully. Continuous casting: The use of a crystallizer and end electromagnetic stirring can promote the homogenization of sulfide composition, reduce dendrite segregation, and make the sulfide distribution more uniform. The first group of ten continuously cast billets are controlled according to the old process, with the secondary water cooling ratio controlled at 0.4 L / Kg. The second group of ten continuously cast billets are controlled according to the new process, with the secondary water cooling ratio controlled at 0.26 L / Kg. The billet serial number is marked as 1-20. The continuous casting billet size is 320mm*425mm*9000mm.

[0032] Comparative Example The first batch of continuously cast billets with a temperature of ≥600℃ were placed in the pit for slow cooling. After a slow cooling time of 120 hours, they were placed in the heating furnace for heating at a temperature of 50℃. The continuous casting billet is heated by a walking beam furnace with a secondary heating temperature of 1140℃ and a soaking temperature of 1150℃. High-pressure water descaling, descaling pressure 23MPa; A two-roll reversible rolling mill (φ1100*2500) is adopted, with a roll cooling water flow rate of 200m³ / h. 3 / h control, the billet mill rolls 11 passes, the billet size is 205*205, the gap between the wear plate of the billet mill pusher and the workpiece is controlled at 5mm at the inlet end and 15mm at the outlet end, and the side vertical rollers of the billet mill pusher are removed. #1 hydraulic shearing head, cutting length 100mm, removes end defects of rolled billet; The intermediate rolling mill consists of six continuous rolling mills arranged alternately in horizontal and vertical configurations. The mills are composed of φ750x6 mills and are all fifth-generation short-stress mills. The rolling temperature is 870℃ and the workpiece size is Φ120mm. Remove the head and tail defects. Shear section: Φ120mm, shearing temperature: 870℃, head length 120mm, tail length 120mm. The finishing mill consists of four continuous stands arranged in an alternating horizontal and vertical configuration. Each mill comprises four φ550mm sections and is a fifth-generation short-stress mill. Rolling temperature: 860℃; round bar specification: Φ100mm; finished product dimensional tolerances are controlled according to national standard group two. Shear section of No. 2 flying shear: Φ100mm, shearing temperature: 860℃, multiple length segmentation of No. 2 flying shear; Grouping: Group the flying shears and multiple rulers; three Φ1800mm metal saws (saw #1, moving saw, saw #2) cut into sections, sawing temperature: 700℃-800℃; Cooling bed, air cooling, straightening; slow cooling before entering the pit, pit temperature ≥400℃; slow-cooled material exiting the pit; straightening; flaw detection, magnetic flux leakage: 0.2mm, initial magnetic flux leakage test pass rate of round steel is shown in Table 1; bundling and warehousing; grinding wheel peeling, ¢180 twelve-head round steel peeling machine, grinding wheel: ¢500*100*203, magnetic flux leakage flaw detection for peeling of defective material; flaw detection, re-testing of peeled and ground round steel, magnetic flux leakage: 0.2mm, comprehensive test pass rate is shown in Table 1; bundling and warehousing.

[0033] Table 1. Initial pass rate and peeling depth detection data of round steel bars in Comparative Example 1

[0034] Example 1 Slowly cool the second batch of steel billets: steel billets with a temperature of ≥600℃ are put into the pit for slow cooling. After slow cooling time of ≥24 hours, they are put into the heating furnace for heating at a temperature of 340℃. The continuous casting billet 320*425*9000 is heated by a walking beam furnace. The secondary heating temperature is 1220℃, the soaking temperature is 1200℃, and the secondary heating temperature is 20℃ higher than the soaking temperature. High-pressure water descaling, with a descaling pressure of 18MPa; A two-roll reversible rolling mill (φ1100*2500) is adopted. The cooling water flow rate of the rolls is controlled at 110-130m3 / h. The billet mill rolls 11 passes. The billet size is 205*205. The gap between the wear-resistant plate of the billet mill pusher and the workpiece is controlled at 25mm at the inlet end and 35mm at the outlet end. Side vertical rollers are added to the billet mill pusher. Two side vertical rollers are installed on each of the front and rear pushers to ensure that the side vertical rollers are 4mm higher than the wear-resistant plate. #1 hydraulic shearing head, cutting length 100mm, removes end defects of rolled billets; The six continuous rolling mills in the intermediate rolling mill are all arranged alternately in horizontal and vertical configurations. The mills consist of φ750x6 mills and are all fifth-generation short-stress mills. The rolling temperature is 920℃ and the workpiece size is Φ120mm. #1 Flying Shear, cutting section: Φ120mm, cutting temperature: 910℃, head cutting length 120mm, tail cutting length 120mm, removes head and tail defects; The finishing mill consists of four continuous rolling mills arranged alternately in horizontal and vertical configurations. The mills are composed of φ550x4 and are all fifth-generation short-stress mills. The rolling temperature is 910℃, the round bar specification is Φ100mm, and the finished product dimensional tolerance is controlled according to the upper limit positive tolerance of the second group of national standards. No. 2 flying shear shear section: Φ100mm, shearing temperature: 900℃, No. 2 flying shear multiple length segmentation; Grouping: Group the flying shear multiples together.

[0035] Three Φ1800mm metal saws (saw #1, mobile saw, and saw #2) are used for sawing and segmentation. The sawing temperature is 760℃-850℃. Cooling bed, air cooling, straightening; slow cooling before entering the pit, pit temperature ≥400℃; slow-cooled material exiting the pit; straightening; flaw detection, magnetic flux leakage: 0.2mm, initial magnetic flux leakage test pass rate for round steel is shown in Table 2; bundling and warehousing; grinding wheel peeling, ¢180 twelve-head round steel peeling machine, grinding wheel: ¢500*100*203, magnetic flux leakage test for flawed material peeling; flaw detection, re-testing of peeled and ground round steel, magnetic flux leakage: 0.2mm, comprehensive test pass rate is shown in Table 2; bundling and warehousing.

[0036] Table 2. Initial pass rate and peeling depth detection data of round steel in Example 1

[0037] Analysis of the data in Tables 1 and 2 shows that the initial pass rate of magnetic flux leakage detection for ￠100 and V2908 round steel in Group 1 was 65.6%, with a scrap rate of 12.4%. For Group 2, the initial pass rate was 90.2%, with a scrap rate of 0.2%. Compared with the original process, the present invention improved the initial pass rate of magnetic flux leakage detection for ￠100 and V2908 round steel by 24.6% and reduced the scrap rate by 12.2%. This invention can improve the initial pass rate of magnetic flux leakage detection, reduce the scrap rate of round steel, reduce the amount of defective round steel peeling, improve the peeling efficiency of the grinding wheel, shorten the finishing cycle, and ensure the finished product delivery time. This invention can be promoted and applied in similar production lines for special steel.

[0038] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for manufacturing free-cutting non-rounding steel, characterized in that, as follows: During the continuous casting of steel billets, the water volume of the secondary water cooling system is controlled at 0.26 L / Kg; During the slow cooling stage of the billet: the billet is placed in the pit at a slow cooling temperature of ≥600℃ and the slow cooling time is ≥24 hours. Then it is placed in the heating furnace for heating, and the temperature in the heating furnace is ≥300℃. During the billet heating stage: control the secondary heating temperature at 1220℃±20℃, and control the homogenization temperature at 1200±20℃; During the high-pressure water descaling stage: the high-pressure water descaling pressure is controlled at 18±1MPa; During the rolling stage: the cooling water flow rate is 110-130 m³ / h. 3 / h control reduces stress cracks generated during the cold-water-deficient rolling process; The gap between the wear-resistant plate and the rolled piece on the billet rolling mill pusher should be controlled at 20-30mm at the inlet end and 30-40mm at the outlet end.

2. The method for manufacturing free-cutting non-rounding steel as described in claim 1, characterized in that, During the billet heating stage, the secondary heating temperature is higher than the homogenization temperature.

3. The method for manufacturing free-cutting non-rounding steel as described in claim 1, characterized in that, During the rolling stage, side rollers are added to the billet rolling press, with two side rollers installed on each of the front and rear presses, ensuring that the side rollers are 3-5mm higher than the wear-resistant plate.

4. The method for manufacturing free-cutting non-rounding steel as described in claim 1, characterized in that, During the rolling stage, the finished product rolling dimensional tolerance is controlled according to the upper limit positive tolerance of the second group of national standards.

5. The method for manufacturing free-cutting non-rounding steel as described in claim 1, characterized in that, During the rolling stage, the rolling temperature of both the intermediate rolling and finishing rolling is ≥900℃.

6. The method for manufacturing free-cutting non-rounding steel as described in claim 1, characterized in that, After rolling is completed, the shearing temperature of the flying shear is controlled to be ≥900℃.

7. A free-cutting non-rounding steel, characterized in that, It is obtained by the manufacturing method of free-cutting non-rounding steel according to any one of claims 1-6.

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